Serena Williams has reached the semifinals at Wimbledon for the 11th time in her career. (Peter Nicholls/Reuters)

Three years younger, three inches shorter and far less powerful, Marion Bartoli knew she had to do something extraordinary to have a chance against Serena Williams in their fourth-round meeting at Wimbledon in 2011.

So Bartoli decided not to take anything off her second serves. If she missed her first serve, she’d simply blast another serve with all her might, rather than a high-percentage second attempt, and not worry about double-faulting. As she figured, she would lose the point anyway if she gifted Williams a weak second serve. Why not take a risk?

“If you serve under a certain speed, she’s just going to expose that constantly and [gain] so much confidence that it’s pointless to do so,” the now-retired Bartoli recalled Tuesday, when asked how she had managed to upset Williams, then the tournament’s four-time and defending champion, in straight sets.

In doing so, Bartoli joined an exclusive club as one of just nine women (all but two now retired) to beat Williams on the grass at Wimbledon.

Tuesday, as Bartoli sized up what remains of a 2018 women’s field that lost all of its top 10 seeds to early-round upsets, she saw no plausible outcome other than a Williams victory.

“She is not a human; she is just a hero,” Bartoli said of Williams, who’s seeking her eighth championship at the All England Club and 24th Grand Slam title just 10 months after giving birth to her first child. “It’s really difficult to [compete] with a hero.”

Italy’s unseeded Camila Giorgi was the latest to try in Tuesday’s quarterfinals on Centre Court, employing high-risk tactics similar to Bartoli’s seven years prior.

The Italian blasted her serve with abandon, taking little off the pace on her second serves, and attacked short balls, seemingly disinterested in any margin for error. It worked in her favor in the early-going: The 52nd-ranked Giorgi, 26, became the first to take a set off Williams all tournament.

But as she has done at each stage here, in just her fourth tournament back since maternity leave, Williams elevated her play.

Through her four Wimbledon matches last week, Williams, 36, carefully had calibrated the speed of her serves, taking care not to aggravate the injured pectoral muscle that had forced her withdrawal midway through the French Open last month.

On Tuesday, trailing the surprisingly hard-serving Giorgi by a set, Williams fired a 122-mph serve (her fastest all tournament) to start the second set, as if to declare she had no intention of being bounced from Wimbledon just yet.

“I went for it,” Williams said of the serve afterward. “I’m like, ‘I’m going as hard as I can on this one.’ ”

She glanced at the monitor that displays service speeds. “Good,” she thought to herself.

Then she did a mental check of her right arm: better than good.

“My arm is, like, amazing,” Williams said.

Giorgi didn’t back off, but the momentum shifted.

“Come on!” Williams screamed after clinching the second set with a service winner to level the match at one set apiece.

Then she broke Giorgi early in the third set and stood firm, earning a spot in Thursday’s semifinal — the 11th Wimbledon semifinal of her career — with a 3-6, 6-3, 6-4 victory.

In doing so, Williams extended her unbeaten streak at Wimbledon to 19 matches and set up a semifinal meeting with 13th seed Julia Goerges of Germany, who dismissed good friend Kiki Bertens of the Netherlands.

Germany’s Angelique Kerber, the highest women’s seed remaining (11th), ousted Daria Kasatkina of Russia in straight sets to advance to Thursday’s other semifinal, in which she’ll face Latvia’s Jelena Ostapenko, who topped Dominika Cibulkova in straight sets.

Former British No. 1 Anne Keothavong, who is among the roughly four dozen retired pros who returned to Wimbledon for this year’s Legends invitation doubles matches, had no answers when asked what it would take to beat Williams this fortnight.

“They know they have to play their absolute best if they want to stand a chance against her — and hope that she has a really bad day,” Keothavong, 34, said with a smile.

The pressure Williams puts on opponents, particularly on grass, which accentuates the power and punch of her serve, is profound.

While some BBC analysts applauded Giorgi’s go-for-broke approach Tuesday, nine-time Wimbledon champion Martina Navratilova, who’s covering the tournament for Tennis Channel, isn’t convinced that answering power with power is the best approach against Williams.

An artful practitioner of serve-and-volley tennis, Navratilova shrewdly varied her shots, particularly on Wimbledon’s grass, during her Hall of Fame career.

That’s how she would take on Williams, if they were contemporaries.

“I would try to give her shots that she doesn’t like: the slice, the little dink,” said Navratilova, 61. “. . . I would try to go around her more, rather than through her.”

But there’s little point in recommending this approach, Navratilova added, because so few female players have a varied arsenal at their command. Virtually all are coached since childhood to blast ball after ball from the baseline, she noted. And that, she believes, is partly what’s behind the high-profile upsets in the women’s draw.

“Because everybody plays the same way now, it’s just whoever is better that day,” Navratilova said. “When one player is a little off now, they can lose because there is no Plan B. They don’t have options.”

Against an opponent as formidable as Williams, simply trying to beat her at a power game is a tall order.

“I think when they play Serena, they try to play their game, but they’re just not good enough,” Navratilova said. “The ones that hit the ball hard don’t move well enough; the people that move well enough, they don’t hit the ball hard enough. Most of all, you have to be able to return her serve and move well.

“You need the total package. You really can’t have a weakness against Serena.”